This invention relates generally to lighting control circuits and more particularly concerns control circuits for use with incandescent lamps powered by a D.C. source.
Low voltage D.C. lighting systems, such as those used in a wide variety of land, water and aircraft, are controlled by on/off switches connecting one terminal of the lamp load to a power source with the other terminal of the lamp load connected to ground. The lighting dimmer circuits presently found in vehicles control the brightness of the instrument panel lights in the vehicle. Brightness is generally controlled by a large potentiometer which varies the voltage delivered to the lamps by converting a considerable amount of energy into heat. One dimmer control circuit presently on the market utilizes a linear voltage regulator to vary the voltage to the lamps and thus vary the brightness. These methods are not only very inefficient but prohibit use of high wattage lamps unless a way to dissipate large amounts of heat is provided. Other systems presently available use two lamps controlled by one double throw or three position switch. In one switch position, both lamps are off; in another, a dim lamp is on; in the third, a bright lamp is on. Such systems have no flexibility and require the use of light fixtures with two lamps.
Pulse width modulation circuits (PWM) have been developed for controlling A.C. lamp circuits supporting both fluorescent and incandescent lamps and lamps powered by a pulsating D.C. source. However, these circuits are generally of the non-switching type, operating only in the PWM mode, and are relatively expensive.
It is, therefore, an object of this invention to provide a PWM dimmer for use with incandescent lamps powered by a low-voltage D.C. source. A further object of this invention is to provide a PWM dimmer with full on/off capability as well as a substantially full range of brightness level control. Another object of this invention is to provide a PWM dimmer that can be used to directly replace the on/off switches commonly used in modern motor vehicle wiring systems. Yet another object of this invention is to provide a PWM dimmer that is highly efficient, operating at relatively low frequencies. It is also an object of this invention to provide a PWM dimmer that is small, inexpensive and reliable. Furthermore, it is among the objects of this invention to provide a PWM dimmer which is user variable or automatically variable to control brightness.